Flatulence is caused by bacteria that live in the large intestine that break down food not already digested higher in the gut. The gas the bacteria produce is emitted from the body as a fart.

Granito and colleagues found that adding these two gut bacteria to black beans, or Phaseolus vulgaris, before cooking makes them even less likely to cause flatulence.

"Legumes, and particularly Phaseolus vulgaris, are an important source of nutrients, especially in developing countries," Granito's team writes.

"In spite of being part of the staple diets of these populations, their consumption is limited by the flatulence they produce."

Smart cooks know they can ferment beans, and make them less gas-inducing, by cooking them in the juices from a previous batch.

But Granito's team wanted to find out just which bacteria were responsible.

Cooking up a stormWhen the researchers fermented black beans with the two bacteria, they found it decreased the soluble fibre content by more than 60% and lowered levels of raffinose, a compound known to cause gas, by 88%.

They fed the beans to rats and then analysed the rats' droppings to ensure that the beans were digested and kept their nutritional value.

When pre-soaked in L. casei, the beans stayed nutritious and produced few gas-causing compounds, the researchers report.

"Therefore, the lactic acid bacteria involved in the bean fermentation, which include L. casei as a probiotic, could be used as functional starter cultures in the food industry," the researchers write.

"Likewise, the cooking applied after induced fermentation produced an additional diminution of the compounds related to flatulence."